This invention relates to aqueous copolymer solutions which thicken, modify rheology, stabilize, and otherwise improve the physical properties of latex systems and dried film resulting from application of latex systems to substrates. Of particular importance is the increase in tensile strength of dry latex films provided by the addition of the novel copolymer solutions to latex coating compositions.
Various polymeric materials have been used to thicken latex formulations, improve their mechanical stability, suspend added fillers, and to impart to the formulations the required flow properties and viscosity for application, as well as impart desirable properties to dried films and coatings after application. Sodium polyacrylate solutions resulting from hydrolysis of polymethyl acrylate are used as thickeners, especially in styrene-butadiene latex compositions containing calcium carbonate and/or kaolin fillers. Such polyacrylate solutions are generally supplied at a viscosity low enough for pumping from tank trucks and for rapid mixing into the latex compositions, yet they are cost effective in thickening latex compositions. They fill a need in highly-automated upholstery fabric backing and carpet backing latex adhesive applications.
The alkali reactive type of thickeners, a typical example of which is disclosed in U.S. Pat. No. 3,070,561, are copolymer emulsions in which the copolymer chain contains both ester and carboxyl groups. Neutralization of the carboxyl groups by addition of alkali at ambient temperature leaves the ester group unaffected, but converts the emulsion to a relatively clear alkali metal or ammonium salt solution of greatly increased viscosity.
Recent improvements of alkali reactive thickeners are disclosed in U.S. Pat. Nos. 4,351,754, 4,384,096, 4,514,552, 4,600,761 and 4,616,074. Such improvements result from inclusion of certain surfactant monomers in these associative copolymer thickeners. Such thickeners and their thickening mechanism are discussed in "Water-Soluble Polymers" by J. E. Glass, Editor, American Chemical Society, Washington, D.C. (1986). These products have been found to be useful in only a limited number of applications, and have been found to be unsatisfactory in the thickening of highly-loaded styrene-butadiene latex compositions used as carpet and fabric backing adhesives which generally are applied by automated methods in carpet and upholstery mills.
Other types of associative thickeners are described in the patent literature. Thus, U.S. Pat. No. 3,708,445 discloses aqueous solution copolymers of certain surfactant monomers with carboxylic acid monomers, to be useful in thickening latex compositions. U.S. Pat. No. 4,138,381 discloses glycol solution terpolymers of certain surfactant monomers with alkyl (meth)acrylate and carboxylic acid monomers to be useful for thickening aqueous polymeric lattices. U.S. Pat. No. 4,268,641 describes copolymers of certain types of surfactant acrylates with carboxyl-containing, ethylenically unsaturated hydrocarbons. Such copolymers generally are polymerized in an organic liquid from which the copolymer precipitates. When base-neutralized the copolymers are useful as thickeners.
In spite of the considerable research activity in this field, the sodium polyacrylate solution thickeners resulting from hydrolysis of polyacrylic acid esters remain the thickeners of first choice for many textile back coating and adhesive applications. These polymers, sometimes crosslinked, have continued in use for almost thirty years without substantial change. However, changes in equipment, compound formulations, and other conditions of use dictate a constant demand for changes and improvements in these polymer thickeners.